During the last several years efforts have been made to replace incandescent lamps by low pressure discharge tubes, the term "replace" here being understood to mean a direct interchangeability. There thus have been proposed fluorescent tubes having a standardized base for instance of the screw-in or bayonet type which can be directly inserted into a standardized socket. Since such a tube may not be directly connected to the network, if only because of the high voltage necessary to trigger the arc in the lamp as well as the necessity to limit lamp current following triggering, the low pressure lamps commercially available are far bulkier and heavier than the incandescent lamps which they are supposed to replace As evidence thereof one may read with interest the patent document GB-A-2 072 942 which describes an example of a low pressure lamp including a heavy inductive ballast enclosed within a translucent housing comprising two U formed fluorescent tubes. The entire structure is terminated by a base of the type E 27 which is screwed directly into a socket of the type which may also receive an ordinary incandescent lamp.
In order to minimize the difficulty presented by the weight and partially that of the bulk, there has been proposed an electronic circuit which transforms the low frequency of the distribution network (sole energization of the lamp in the above-cited document) into a high frequency for energizing the tube. This is the case in the example of the lamp shown on the drawing in FIG. 1 which illustrates a lamp sold by the Osram Company under the registered trademark "DULUX EL" The energization at high frequency (on the order of 35 kHz) enables a reduction in the size of the passive components such as that of the current limiting inductive ballast. Thus, the lamp 1 shown on FIG. 1 includes a fluorescent tube 2 and a base 3, tube and base being united by an intermediate element 4. The tube is of a well-known type such as described for instance in the patent document GBA-2 050 046. The intermediate element 4 in addition to mechanically coupling the tube to the base contains an electronic circuit for raising the energization frequency as well as a ballast and a starter for limiting the current in the tube and respectively triggering the arc. The intermediate element likewise contains an arrangement for eliminating radio-frequency parasitic noise. This latter arrangement mainly includes a double self coupled by a ferromagnetic core arranged in series with the energization of the electronic circuit. Such double self, although not reaching the weight and bulk of the ballast employed for direct energization from the network, nevertheless occupies a relatively substantial volume which necessitates an intermediate element 4 of relatively large diameter in any case double that exhibited by the base.
Efforts have been made to further diminish the bulk of the intermediate element 4. Such is the case for the fluorescent lamp described in the international patent application WO 88/03 702. Here the energization circuit for the tube is entirely confined within the standardized base of the lamp However, this energization lacks an electronic frequency converter and the lamp is energized at the frequency of the distribution network (50 or 60 Hz). The ballast is a simple capacitor shunted by a high value resistor. As example, one may cite the value of 2 .mu.F for the capacitor and 150 k.OMEGA. for the resistor. The inventor of the lamp described in document WO 88/03 702 indicates moreover that a transistorized electronic ballast is highly fragile and occupies substantial volume which leaves one to conclude that it will not be capable of being housed in the lamp base. It will be shown in the description to follow that this conclusion is not correct and that it is perfectly possible in the present state of the art and with a good margin of security to confine an electronic energization circuit within the lamp base. On the other hand, the arrangement suggested in the last cited document includes several difficulties. It exhibits initially a very low power factor (cos.phi. substantially less than 0.2) which can be compensated only by a large volume self which in any case would be impossible to place within the base. Numerous countries will forbid the sale of such a lamp if the power factor is not compensated Furthermore, the ballast capacitor typically of two .mu.F must withstand almost the entire network voltage This requires a large volume capacitor, one which in any case would be provided for energization at 220 VAC and which may not be placed within the lamp base without assuming risks inacceptable in certain countries where safety regulations must be observed to avoid prohibition of sale.